Why does a wizard not do this ?!
For a couple reasons really, a large one being they are aimed a DMs and do not have an LA entry.
But Riddled's +6 Int is offset by -4 Wisdom, -1 Spell Slot of each Spell level, a weakness that allows anyone to make an opposed Knowledge Check to Fascinate for 1d10 rounds, and if it's ever hit with Maze it needs three Int checks to escape instead of one. It's not exactly a good thing to use.
Spellhording is mostly pretty positive, you get +2 Int, -4 Wis, & +5 Racial to Spellcraft; Eschew Materials, Scribe Scroll, & a form of Improved Counterspell; plus you use your body as your Spellbook and in a pinch you can even blow your "pages" as Scrolls or even pay for Material/XP Components. So when you rip the flavor and the fact it's a psychosis out it's mechanically pretty powerful. But it is expressly a mental condition that's paired with a desire to learn rare and impractical Spells above all else. It's a fancy way of saying your True Dragon would rather learn Entangling Dragonmark instead of Magic Missile. You also forgo any ability to build a base of operations and overvalue your worthless Spells which is kind of a pain in the rear to fortify anything or trade Spells. Finally a Spellhording Dragon's Treasure Entry doesn't allow the Dragon to buy/have Magic Items in favor of stockpiles of Scrolls which can be taken in a fairly debilitating way when you're trying to port it over for PC usage.